Alloy solder connect assembly and method of connection

Abstract

An electronic assembly (100) includes multiple solder connections (101) coupling faying surfaces (202, 204) of two substrates (106, 108). A solder connection (201) is fabricated by reflow heating of a first, less compliant solder paste (312, 314) and a compliant preform (210) so as to cause compliant material of the preform (210) to dissolve into the solder paste (312, 314). Upon solidification of the solder paste (312, 314), blended regions (520, 522), having a gradual, changing concentration of compliant material form between resulting fillets (212, 214) and the preform (210). The blended regions (520, 522) transfer temperature induced shear stresses, caused by thermal cycling of the electronic assembly (100), from the fillets (212, 214) into the compliant preform (210).

We claim: 
     
       1. A method of forming a melted alloy connection to a faying surface of a substrate, the method comprising the steps of: (a) depositing a first alloy solder having a first melting point onto the faying surface;   (b) placing a second alloy solder formed of a material more compliant than the first alloy solder in contact with the first alloy solder, the second alloy solder having a second melting point greater than the first melting point; and   (c) heating the first alloy solder and the second alloy solder to a temperature between the first melting point and the second melting point sufficient to dissolve the material more compliant than the first alloy solder from the second alloy solder into the first alloy solder and transform the first alloy solder into a third alloy solder that is elementally different from the first alloy solder.   
     
     
       2. A method of forming a melted alloy connection to a faying surface of a substrate, the method comprising the steps of: (a) depositing a first alloy solder having a first melting point onto the faying surface;   (b) placing an indium alloy solder in contact with the first alloy solder, the indium alloy solder having a second melting point greater than the first melting point; and   (c) heating the first alloy solder and the indium alloy solder to a temperature between the first melting point and the second melting point sufficient to dissolve indium from the indium alloy solder into the first alloy solder and transform the first alloy solder into a third alloy solder that is compositionally different from the first alloy solder and the indium alloy solder.   
     
     
       3. The method according to claim 2, wherein the first alloy solder deposited in said step (a) is substantially indium-free. 
     
     
       4. The method according to claim 3, wherein the third alloy solder comprises a second indium alloy solder. 
     
     
       5. The method according to claim 1, wherein the first alloy solder deposited in said step (a) is a paste; and wherein the second alloy solder placed in said step (b) is a preform. 
     
     
       6. The method according to claim 1, further comprising the step of: (d) cooling the melted alloy connection to solidify the third alloy solder into a multiphase mixture of elements of said second alloy solder and said first alloy solder.   
     
     
       7. The method according to claim 1, wherein said step (c) comprises the substep of: (c1) heating the first alloy solder and the second alloy solder to a temperature above the first melting point for about 15 seconds to about 60 seconds, wherein transforming the first alloy solder into the third alloy solder occurs between 0 seconds and about 60 seconds.   
     
     
       8. The method according to claim 1, wherein the first melting point is lower than the second melting point by a temperature of about 25° C. or more. 
     
     
       9. The method according to claim 2, wherein the first alloy solder is a material selected from a group consisting of SnPb, SnPbAg, and SnBiPbCd and the indium alloy solder is a material selected from a group consisting of InPb and InAg. 
     
     
       10. The method according to claim 2, wherein the first alloy solder consists of amounts of tin and amounts of lead sufficient to cause the first melting point and the indium alloy solder consists of amounts of lead and amounts of indium sufficient to cause the second melting point, the second melting point being about 25° C. or more above the first melting point. 
     
     
       11. The method according to claim 2, wherein the first alloy solder consists of amounts of tin and amounts of lead sufficient to cause the first melting point and the indium alloy solder consists of amounts of silver and amounts of indium sufficient to cause the second melting point, the second melting point being from about 25° C. above the first melting point. 
     
     
       12. The method according to claim 2, wherein the indium alloy solder comprises between about 3 and 50 weight percent indium and the balance substantially lead. 
     
     
       13. The method according to claim 2, wherein the first alloy solder comprises about 63% tin and 37% lead, the indium alloy solder comprises about 81% lead and 19% indium, and the third alloy solder comprises about 42% tin, 40% lead, and 18% indium. 
     
     
       14. The method according to claim 2, wherein the first alloy solder comprises about 63% tin and 37% lead and the indium alloy solder comprises about 75% lead and 25% indium. 
     
     
       15. The method according to claim 2, wherein the first alloy solder comprises about 35.5% bismuth, 35% lead, 20% tin, and 9.5% cadmium and the indium alloy solder comprises about 81% lead and 19% indium. 
     
     
       16. The method according to claim 2, wherein the first alloy solder comprises about 35.5% bismuth, 35% lead, 20% tin, and 9.5% cadmium and the indium alloy solder comprises about 95% lead and 5% indium. 
     
     
       17. The method according to claim 2, wherein the first alloy solder comprises about 35.5% bismuth, 35% lead, 20% tin, and 9.5% cadmium and the indium alloy solder comprises about 75% lead and 25% indium. 
     
     
       18. The method according to claim 2, wherein the first alloy solder comprises about 35.5% bismuth, 35% lead, 20% tin, and 9.5% cadmium and the indium alloy solder comprises about 50% lead and 50% indium. 
     
     
       19. The method according to claim 2, wherein the indium alloy solder comprises between about 90 and 97 weight percent indium and the balance substantially silver. 
     
     
       20. The method according to claim 2, wherein the first alloy solder comprises about 35.5% bismuth, 35% lead, 20% tin, and 9.5% cadmium and the indium alloy solder comprises about 90% indium and 10% silver. 
     
     
       21. The method according to claim 2, wherein the first alloy solder comprises about 62% tin, 36% lead, and 2% silver and the indium alloy solder comprises about 81% lead and 19% indium, and the third alloy solder comprises about 42% tin, 40% lead, 18% indium, and trace amounts of silver. 
     
     
       22. The method according to claim 2, wherein the first alloy solder comprises about 62% tin, 36% lead, and 2% silver, the indium alloy solder comprises about 90% lead and 10% indium, and the third alloy solder comprises about 64% tin, 32% lead, 4% indium, and trace amounts of silver. 
     
     
       23. The method according to claim 2, wherein the first alloy solder comprises about 62% tin, 36% lead, and 2% silver, the indium alloy solder comprises about 95% lead and 5% indium, and the third alloy solder comprises about 76% tin, 22% lead, 2% indium, and trace amounts of silver. 
     
     
       24. A method of forming a melted alloy connection to a faying surface of a substrate, the method comprising the steps of: (a) depositing a substantially indium-free alloy solder having a first melting point onto the faying surface;   (b) placing an indium alloy solder in contact with the indium-free alloy solder, the indium alloy solder having a second melting point greater than the first melting point; and   (c) heating the indium-free alloy solder and the indium alloy solder to a temperature between the first melting point and the second melting point sufficient to combine indium from the indium alloy solder into the substantially indium-free alloy solder.   
     
     
       25. The method according to claim 24, wherein the substantially indium-free alloy solder deposited in said step (a) is a paste; and wherein the indium alloy solder placed in said step (b) is a preform. 
     
     
       26. The method according to claim 25, wherein the first melting point is lower than the second melting point by a temperature of about 25° C. or more. 
     
     
       27. The method according to claim 25, wherein the substantially indium-free alloy deposited in said step (a) is indium-free.